The present invention relates to a hologram (diffraction grating) and an optical system using such a hologram as a key device.
Recently, the optical storage using an optical disk is being widely used as a high-density and large-capacity recording medium. The reliability of such an optical storage greatly depends upon its optical system used. An optical pickup head apparatus, being a main device of such an optical system, is constructed with a hologram for the size-reduction purposes.
FIG. 12 shows an arrangement of an prior art optical pickup apparatus as described in the Japanese Patent provisional Publication No. 4-212730 (the same applicant as this application). In FIG. 12, a light beam 3 emitted from a light source such as a semiconductor laser 2 is reflected on a reflection type blazed hologram (which will be referred hereinafter to as a hologram) 105 so as to be incident on an objective lens 4 and then focused on an information medium (recording medium) 5. Light reflected from the information medium 5 advances along the same optical path (return optical path) to arrive at the hologram 105. The +1st order diffraction beam 6 due to the hologram 105 in the return optical path is incident on a photodetector 7, thereby obtaining a servo signal and an information signal by the calculation of the output of the photodetector 7. Numeral 61 designates the 0 order diffraction beam in the outward optical path. Here, the hologram 105 is blazed in order to prevent that unnecessary diffraction light due to the hologram 105 in the outward optical path from the light source 2 to the information medium 5 is reflected on the information medium 5 to be incident on the photodetector 7. The blazed hologram 105 may be manufactured as illustrated in FIG. 13 where 29 denotes a hologram base plate, 30 represents a resist, 32A, 32B designate photomasks, 33 depicts a blazed hologram, and h1, h2 represent etching depths. That is, a series of processes including exposure, development, removal of resist and etching are effected two times to form a multi-level hologram having a stair-like configuration in the cross section as illustrated in FIG. 14 where a7=b7=c7=d7. This arrangement can improve the degree of freedom for control of the diffraction efficiency. That is, the first etching depth h1 and the second etching depth h2 are controlled independently of each other so as to increase the light utilization efficiency in the outward and return optical paths concurrently with decreasing the diffraction efficiency of the unnecessary diffraction beam.
The aforementioned blazed hologram 105 also acts as a mirror to bend the optical axis and hence it is possible that an optical pickup head apparatus can be constructed with a small number of parts, and further the above-described arrangement can omit a collimator lens, thus reducing the size, weight and cost of the optical pickup head apparatus.
However, the incident angles of light are different between portions indicated at characters A and E in FIG. 12, and hence a difference in the diffraction efficiency of the diffraction beam due to hologram 105 occurs. Here, the reason of the occurrence of the difference in the diffraction efficiency of the diffraction beam due to a reflection type hologram will be described hereinbelow with reference to FIG. 15. In FIG. 15, references 65a, 65b are incident light beams on the hologram, 66a, 66b represent diffraction beams, .theta.1 denotes an incident angle, .theta.2 depicts a diffraction angle, 33a, 33b designate the upper and lower side reflecting surfaces of a relief and reflection type blazed hologram, h represents a difference between the upper and lower side reflecting surfaces of the relief and reflection type blazed hologram, and L1, L2 are optical path differences due to the difference h. At this time, the optical path differences becomes L1=h.multidot.cos .theta.1 and L2=h.multidot.cos .theta.2. As obvious from these equations, the optical path differences are respectively proportional to the cosine of the incident angle .theta.1 and the cosine of the diffraction angle .theta.2, and hence, when the incident angle .theta.1 and diffraction angle .theta.2 are set to 45.degree., the optical path differences sharply vary in response to variations of the incident angle .theta.1 and diffraction angle .theta.2 whereby the diffraction efficiency varies. In the above-described conventional optical pickup head apparatus, a servo signal offset generates in response to the variation of the diffraction efficiency or the frequency characteristic of the RF signal deteriorates.